Technical Field
The present disclosure relates to a process for manufacturing a semiconductor device comprising an empty trench structure, and a semiconductor device obtained with said process.
Description of the Related Art
In the present context, the term “empty trench” refers to the fact that the trench (or some other cavity of any shape) is not filled, irrespective of the conditions of pressure existing inside the trench itself.
In semiconductors devices it is at times required to provide an empty trench. For example, empty-channel transistor devices are under study (also called “microminiature vacuum tubes” or “vacuum microelectronic devices”—VMDs) present in which is a very deep trench closed at the top by a layer of metal, for example aluminum, which operates as ion-emitter element.
An example of an embodiment of an empty trench microelectronic device and its manufacturing method are described, for example, in U.S. Patent Publication No. 2014/0353576 in the name of the present applicant, as described hereinafter.
With reference to FIG. 1, an empty trench device 1 comprises a substrate 2 of heavily doped semiconductor material, such as silicon, a stack 3 of layers 4-6, which extends above the substrate 1, a trench or hole 10, which extends throughout the thickness of the stack 3, as far as the substrate 1, and a cathode metal region 11, which extends above the stack 3 and closes the trench 10 at the top. The trench 10 is here in a condition of negative pressure, and is thus defined as “vacuum hole”.
The stack 3 of layers here comprises a first insulating layer 4 on the substrate 2, a semiconductor layer 5, made, for example, of polycrystalline silicon, and a second insulating layer 6, on the semiconductor layer 5.
A contact structure 12 is formed above the cathode metal region 11, and an anode metal layer 13 extends underneath the substrate 2.
A passivation layer 15, of silicon nitride, coats the side walls of the trench 10.
The device 1 is obtained as follows: the layers 4-6 are deposited in sequence on the substrate 2; then, using a resist mask, the layers 4-6 are chemically etched in sequence in different apparatuses and using appropriate etching solutions. Next, the passivation layer 15 is deposited, in a highly conformable way, and is then removed from the bottom of the trench 10 and from the external part of the trench 10, above the second insulating layer 6. Then a metal layer, made for example of aluminum, that closes the trench 10 at the top and forms the cathode metal region 11 is deposited in a non-conformable way and shaped lithographically.
In the practical manufacture of the device, there have been noted difficulties in deposition of the metal layer that is to form the cathode metal region 11. In fact, even using non-conformable material and deposition techniques, it is not always possible to guarantee that the metal does not penetrate extensively within the trench 10. Furthermore, the presence of metal particles inside the trench is disadvantageous given that any possible metal traces in the trench 10 may give rise to leakage that cannot be easily distinguished from emissions of the cathode metal region, resulting in an improper operation of the device.
It is thus desirable for the metal layer (which constitutes the cathode region) to extend above the trench and not penetrate therein.
This requirement, also in common with other empty trench semiconductor products, is not easy to meet, given the absence of a stop structure, also taking into account the conditions of negative pressure present in certain applications.